The present invention relates to a method for generating an amplitude-modulated transmission signal with an adjustable mean transmission power, in which an amplitude modulation signal is applied to an input of a transmission amplifier and in which the amplitude modulation signal is regulated by using a feedback component of the transmission signal. In addition, the present invention relates to a corresponding circuit and to a communication terminal including such a circuit.
In most transmitters, it is required that the transmission power can be adapted to the required range and possibly to the radio conditions in the respective environment of the transmitter and receiver, respectively. In the mobile radio standard GSM, the so-called power control level serves this purpose. This is used for adjusting the mean transmission power of the signal to be sent out, the “transmission power level”. To avoid adjacent-channel interference caused by burst-mode operation, furthermore, the transmission power is slowly raised from zero to the target value at the beginning of the burst and is slowly reduced at the end of the burst. This so-called “power ramping” and the power control level are examples of the expressions mean transmission power, transmission power level or transmission output stage used in the text which follows. These expressions are not restricted to a graduated adjustment but continuous adjustment of the power value is also possible. Similarly, the present invention described hereinafter is not restricted to mobile radio applications.
In an amplitude-modulated signal, the instantaneous transmission power present at a particular point in time depends on the current amplitude of the signal. As a rule, the information to be transmitted is converted into amplitude modulation in such a manner that for certain information chains in a control system, digital samples are determined for the amplitude and these digital samples are then used for generating an analog amplitude modulation signal. The samples for the most varied chains of information (e.g., transmit data) are in each case stored, for example, in a table in a ROM memory. For reasons of saving memory space, only the samples for a precisely defined mean transmission power are stored. Depending on the mean transmission power required, therefore, the samples for the required mean power are calculated from these samples via suitable scaling. Therefore, the samples thus calculated or scaled, respectively, are used for generating the amplitude modulation.
The amplitude modulation signal is usually regulated by using a feedback component of the transmission signal in order to obtain a stable pure amplitude modulation signal. The regulation receives the samples, or values derived therefrom, as nominal values. The nominal values are compared with the part of the transmission signal, tapped off the output of the transmission amplifier, which represents the current actual value, and the transmission amplifier, also-called output stage in the text which follows, is correspondingly corrected. The dynamic range of the nominal signal is composed of the dynamic range of the amplitude modulation itself and the range of the possible adjustable transmission power. In the EDGE standard, the dynamic range of the amplitude modulation is 17 dB and the transmission power can be adjusted over a range of 27 dB. The overall dynamic range is, therefore, 44 dB. The same dynamic range also occurs in the actual signal. The feedback or control loop, respectively, must be able to handle this dynamic range. The requirement for the dynamic range can be even greater in other transmission methods; e.g. in the CDMA standard.
Such a wide dynamic range requires an elaborate circuit for linearizing the characteristics of the components needed in the control loop. In addition, a very efficient limiter circuit is needed for a possibly additional phase control loop in order to feed back the transmission signal for regulating the phase-modulated signal component when generating a phase-modulated and amplitude-modulated transmission signal.
WO 86 00476 A discloses a method and a device for generating an amplitude-modulated transmission signal with an adjustable mean transmission power, in which an amplitude modulation signal is applied to one input of a transmission amplifier and in which the amplitude modulation signal is regulated by utilizing a feedback component of the transmission signal, the component of the transmission signal fed back being utilized for the regulation attenuated in accordance with the required mean transmission power.
It is an object of the present invention to specify a method and a circuit for always retaining the stability while reducing the requirement for the dynamic range and for enabling simple and inexpensive components to be used.